Approximately half of all heart failure cases occur in patients with preserved systolic function, making diastolic heart failure a substantial health problem. The prevalence of diastolic dysfunction has been increasing, the mortality rate is roughly equivalent to that of patients with systolic heart failure, and, unlike in systolic heart failure, mortality has been unaffected by current therapies. In part, this stems from a lack of mechanistic understanding about this condition. Nevertheless, hypertension is among the leading risk factors for diastolic dysfunction, and this application will explore a possible mechanism whereby hypertension may cause diastolic dysfunction. Nitric oxide (NO)has been shown to enhance diastolic relaxation. Hypertension results in an oxidative stress that leads to loss of NO through nitric oxide synthase (NOS) uncoupling, a condition characterized by the loss of tetrahydrobiopterin (BH4). BH4 is a critical co-factor in the NOS electron transport chain. Uncoupled NOS occurs when BH4 becomes oxidized. When uncoupled, NOS produces superoxide rather than NO. Therefore, a small amount of oxidant stress can lead to a positive feedback loop that ultimately depletes most NO synthetic capacity. Recently, we have shown that BH4 is preferentially oxidized by peroxynitrite and that NAD(P)H oxidase (NOX)-derived peroxynitrite is central to NOS uncoupling. Angiotensin II (Angll) is known to activate NOXs, which appear to be a source for the initial oxidant stress resulting in progressive NOS uncoupling, suggesting a role for the renin-angiotensin system in NOS uncoupling. Based on the strong clinical association of hypertension and diastolic dysfunction, we developed a murine model of diastolic dysfunction using mice with deoxycorticosterone acetate-salt induced hypertension. We have shown that this model generates oxidative stress through an NOX-dependent mechanism. In preliminary studies, we show these mice have cardiac oxidative stress, reduced BH4, uncoupled endothelial NOS (eNOS), and diastolic dysfunction by echocardiographic and hemodynamic measures. Dietary supplementation with BH4 prevents or reverses the onset of eNOS uncoupling and diastolic dysfunction, suggesting that NOS uncoupling may play a central role in the pathogenesis of diastolic dysfunction induced by this form of hypertension. Therefore, we hypothesize that persistent overproduction of reactive oxygen species during hypertension results in a vicious cycle where excess reactive oxygen species production leads to oxidative depletion of myocardial BH4and NOS uncoupling, which in turn perpetuates the overproduction of superoxide, further depleting BH4 and reducing myocardial NO bioavailability. A decreased NO bioavailability in the heart results in impaired diastolic relaxation of cardiomyocytes. Furthermore, we will test whether, as in the vasculature, if NOXs are central to generating the reactive oxygen species necessary for initial NOS uncoupling. We will test whether scavenging of reactive oxygen species will prevent BH4 depletion, uncoupled NOS, and diastolic dysfunction. Finally, since NOXs are Angll-dependent, cardiac Angll seems to play a role in diastolic dysfunction, Angll inhibitors ameliorate diastolic dysfunction in a salt-sensitive hypertensive rat model, and a model of cardiac renin-angiotensin system (RAS) shows diastolic dysfunction in the absence of hypertension, we will test whether pharmacological therapy targeting at reducing renin-angiotensin system activation will prevent BH4depletion, uncoupled NOS, and diastolic dysfunction.